pgsql: Add regression tests for remote execution of extension operators

Add regression tests for remote execution of extension operators/functions.

Rather than relying on other extensions to be available for installation,
let's just add some test objects to the postgres_fdw extension itself
within the regression script.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/b9f117d6cd3c79780b0c0e57068f0837f7493aa6

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out |  178 +++++++++++++++++-------
contrib/postgres_fdw/sql/postgres_fdw.sql      |   68 ++++++---
2 files changed, 182 insertions(+), 64 deletions(-)